D R McKenzie

Publications (227)359.59 Total impact

• Article: A method to remove residual signals in fibre optic luminescence dosimeters.

  J J Lee, P Z Y Liu, D R McKenzie, N Suchowerska
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  ABSTRACT: Whenever a fibre optic is used to convey a light signal through a radiation field, it is likely that an unwanted background signal will arise from Cerenkov or fluorescent light which will contaminate the signal. In luminescence dosimetry of high energy beams, when a fibre optic is used to convey the signal from the radiation field to the detector, Cerenkov light is the dominant contributor to the background signal and must be corrected for. In this work, a novel method is demonstrated to separate the signal from the unwanted background. A remotely operated shutter is used to block the signal, allowing the residual background in the fibre optic to be quantified. This background is subtracted from the total measurement acquired in a subsequent irradiation, enabling the luminescence signal to be extracted. Two types of shutter mechanism are considered: an electro-mechanical device to intercept the light path and an LCD device to block the light by cross-polarization. Both shutters were characterized and incorporated into a fibre optic dosimetry system used to measure the radiation dose produced by external beam radiation linear accelerators. The dosimeter using each of the shutters in turn was exposed to a 6 MV photon beam to determine their performance, including the measurement of field size dependent output factors. The mechanical shutter determined the output factors to within 0.29% of those measured with an ionization chamber, whereas the LCD shutter gave results that deviated by up to 2.4%. The switching precision of both shutters was good with standard deviations of less than 0.25% and both were able to completely block the light signal when closed. The use of shutters could therefore be applied to any fibre optic based system to quantify and remove a reproducible background arising from any source including ambient, fluorescent and Cerenkov light.
  Physics in Medicine and Biology 03/2013; 58(5):1581-90. · 2.83 Impact Factor
• Article: Optimization of temporal dose modulation: comparison of theory and experiment.

  J M Bewes, N Suchowerska, L Cartwright, M A Ebert, D R McKenzie
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  ABSTRACT: To compare theoretical predictions and experimental measurements of cell survival after exposure to two different temporally modulated radiation dose patterns that deliver the same dose in the same overall time. The authors derived an analytic expression for the dose protraction factor G in the Lea-Catcheside formalism for cell survival for "triangle" and "V" temporal modulation of dose. These temporal dose patterns were used in experimental clonogenic studies of a melanoma cell line (MM576) and a nonsmall-cell lung cancer line (NCI-H460) that have different alpha, beta, and repair parameters. The overall treatment time and total dose were kept constant. The analytic expressions for G for the two temporal modulations are presented as a function of a single variable, the product of the exposure time, and the repair constant, enabling G to be evaluated for any exposure time and for any cell line. G for the triangle delivery pattern is always the larger. For the MM576 cell line, following a large dose of 6 Gy, a larger survival fraction was found for the V delivery pattern. No difference in survival was observed for lower doses or for the NCI-H460 cell line at any dose. These results are predicted by our theory, using published values of alpha, beta, and repair time within the limits of experimental uncertainty. The study provides evidence to confirm that cell lines having large beta values exhibit a response that is sensitive to the pattern of dose delivery when the delivery time is comparable with the repair time. It is recommended that the dose delivery pattern be considered in hypofractionated treatments.
  Medical Physics 06/2012; 39(6):3181-8. · 2.83 Impact Factor
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  Available from: 217.219.214.30

  Article: Reply to the comment on: 'Plastic scintillation dosimetry: comparison of three solutions for the Cerenkov challenge'

  P Z Y Liu, N Suchowerska, J Lambert, P Abolfathi, D R McKenzie
  Physics in Medicine and Biology 05/2012; 57(11):3667-3673. · 2.83 Impact Factor
• Article: Influence of Gas Entry Point on Plasma Chemistry, Ion Energy and Deposited Alumina Thin Films in Filtered Cathodic Arc

  J. Rosen, P. O. Å. Persson, M. Ionescu, J. Pigott, D. R. McKenzie, M. M. M. Bilek
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  ABSTRACT: The effect of gas entry point on the plasma chemistry, ion energy distributions and resulting alumina thin film growth have been investigated for a d.c. cathodic arc with an aluminum cathode operated in an oxygen/argon atmosphere. Ions of aluminum, oxygen and argon, as well as ions originating from the residual gas are investigated, and measurements for gas entry at both the cathode and close to the substrate are compared. The latter was shown to result in higher ion flux, lower levels of ionised residual gas, and lower ion energies, as compared to gas inlet at the cathode. These plasma conditions that apply when gas entry at the substrate is used result in a higher film deposition rate, less residual gas incorporation, and more stoichiometric alumina films. The results show that the choice of gas entry point is a crucial parameter in thin film growth using reactive PVD processes such as reactive cathodic arc deposition.
  Plasma Chemistry and Plasma Processing 04/2012; 27(5):599-608. · 1.60 Impact Factor
• Article: Characterisation of polystyrene coatings after plasma immersion ion implantation and adsorption of protein

  S Dekker, A Kondyurin, B Steel, M M M Bilek, D R McKenzie, M James
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  ABSTRACT: A polystyrene film spun onto polished silicon substrates was implanted with either nitrogen or argon ions using plasma immersion ion implantation (PIII) and subsequently investigated by X-ray and neutron reflectometry, UV-VIS and FTIR ellipsometry, as well as by FTIR and Raman spectroscopy. The depth profile of the densified carbon structures resulting from the ion collision cascades in the polystyrene coating are clearly observed by both X-ray and neutron reflectometry. Argon ions produce a higher density modified layer at a shallower depth than nitrogen ions. The thickness measured for these graded layers agrees with the expected depths of ion implantation as calculated by SRIM. The sensitivity of X-ray and neutron reflectometry allows resolution of density and hydrogen content gradients within the graphitized layers. The treated layers were found to covalently immobilized protein directly from solution. The tropoelastin protein monolayers immobilized on the surface were characterized. Tropoelastin remained on the surface after SDS washing.
  04/2012;
• Article: Changes in lung tumor shape during respiration.

  E Kyriakou, D R McKenzie
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  ABSTRACT: Evidence that some lung tumors change shape during respiration is derived from respiratory gated CT data by statistical shape modeling and image manipulation. Some tumors behave as rigid objects while others show systematic shape changes. Two views of lung motion are presented to allow analysis of the results. In the first, lung motion is viewed as a wave motion in which inertial effects arising from mass are present and in the second it is a quasistatic motion in which the mass of the lung tissues is neglected. In the first scenario, the extremes of tumor compression and expansion are expected to correlate with maximum upward and downward velocity of the tumor, respectively. In the second, they should occur at end exhale and end inhale, respectively. An observed correlation between tumor strain and tumor velocity provides more support for the first view of lung motion and may explain why previous attempts at observing tumor shape changes during respiration have largely failed. The implications for the optimum gating of radiation therapy are discussed.
  Physics in Medicine and Biology 02/2012; 57(4):919-35. · 2.83 Impact Factor
• Article: Nonequilibrium route to nanodiamond with astrophysical implications.

  N A Marks, M Lattemann, D R McKenzie
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  ABSTRACT: Nanometer-sized diamond grains are commonly found in primitive chondritic meteorites, but their origin is puzzling. Using evidence from atomistic simulation, we establish a mechanism by which nanodiamonds form abundantly in space in a two-stage process involving condensation of vapor to form carbon onions followed by transformation to nanodiamond in an energetic impact. This nonequilibrium process is consistent with common environments in space and invokes the fewest assumptions of any proposed model. Accordingly, our model can explain nanodiamond formation in both presolar and solar environments. The model provides an attractive framework for understanding noble gas incorporation and explains all key features of meteoritic nanodiamond, including size, shape, and polytype. By understanding the creation of nanodiamonds, new opportunities arise for their exploitation as a powerful astrophysical probe.
  Physical Review Letters 02/2012; 108(7):075503. · 7.37 Impact Factor
• Source

  Available from: Giordano Scappucci

  Article: n-Type Doping of Germanium from Phosphine: Early Stages Resolved at the Atomic Level

  G Scappucci, O Warschkow, G Capellini, W M Klesse, D R McKenzie, M Y Simmons
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  ABSTRACT: To understand the atomistic doping process of phosphorus in germanium, we present a combined scanning tunneling microscopy, temperature programed desorption, and density functional theory study of the reactions of phosphine with the Ge(001) surface. Combining experimental and theoretical results, we demonstrate that PH2 + H with a footprint of one Ge dimer is the only product of room temperature chemisorption. Further dissociation requires thermal activation. At saturation coverage, PH2 + H species self-assemble into ordered patterns leading to phosphorus coverages of up to 0.5 monolayers.
  Physical Review Letters 01/2012; 109(7). · 7.37 Impact Factor
• Article: Fuel Selection for Pulsed Cathodic Arc Thrusters

  P. R. C. Neumann, M. M. M. Bilek, D. R. McKenzie
  Journal of Propulsion and Power 01/2012; 28(1):218-221. · 0.76 Impact Factor
• Article: Light propagation in multimoded square hollow waveguides

  P Naseri, D R McKenzie, P Liu, S Fleming, N Suchowerska
  Journal of Optics. 01/2012; 14(10):105703.
• Article: Linker-free Covalent Thermophilic Beta-glucosidase Functionalized Polymeric Surfaces

  S. L. Hirsh, N. J. Nosworthy, A. Kondyurin, C. G. dos Remedios, D. R. McKenzie, M. M. M. Bilek
  Journal of Materials Chemistry 09/2011; · 5.97 Impact Factor
• Article: Plastic scintillation dosimetry: comparison of three solutions for the Cerenkov challenge.

  P Z Y Liu, N Suchowerska, J Lambert, P Abolfathi, D R McKenzie
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  ABSTRACT: In scintillation dosimetry, a Cerenkov background signal is generated when a conventional fibre optic is exposed to radiation produced by a megavoltage linear accelerator. Three methods of measuring dose in the presence of Cerenkov background are compared. In the first method, a second background fibre is used to estimate the Cerenkov signal in the signal fibre. In the second method, a colour camera is used to measure the combined scintillation and Cerenkov light in two wavelength ranges and a mathematical process is used to extract the scintillation signal. In the third method, a hollow air core light guide is used to carry the scintillation signal through the primary radiation field. In this paper, the strengths and weaknesses of each dosimetry system are identified and recommendations for the optimum method for common clinical dosimetry situations are made.
  Physics in Medicine and Biology 08/2011; 56(18):5805-21. · 2.83 Impact Factor
• Chapter: Deposition of Nanoscale Multilayered Structures Using Filtered Cathodic Vacuum Arc Plasma Beams

  M. M. M. Bilek, D. R. McKenzie, T. W. H. Oates, J. Pigott, P. Denniss, J. Vlcek
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  ABSTRACT: Nanoscaled multilayered thin film structures have recently been identified as good candidates for applications where super-tough and super-hard materials are required. The properties of the structures diverge considerably from those of the bulk constituents and thus are likely to depend on the interfaces present. Interfaces in such structures can be smooth, graded or rough. To determine to what extent the interface quality determines the properties it is necessary to have control of the degree of mixing at the interface during deposition. The fully ionized cathodic arc plasma has a relatively narrow natural ion energy distribution as compared with other deposition processes such as sputtering. This affords good control of the impact energy at the growth surface using substrate bias, enabling control of the degree of interface mixing. This paper discusses a dual source pulsed filtered cathodic arc plasma deposition system designed to deposit a variety of multilayered thin film materials with various interface morphologies. We discuss also methods of producing multilayered materials in single source continuous arc systems.
  07/2011: pages 173-186;
• Article: Comment on 'Shear stiffness in nanolaminar Ti3SiC2 challenges ab initio calculations'.

  M F Cover, M M M Bilek, D R McKenzie
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  ABSTRACT: In a recent publication by Kisi et al (2010 J. Phys.: Condens. Matter 22 162202) the authors present experimentally measured elastic constants for the M(n + 1)AX(n) (MAX) phase, Ti(3)SiC(2), that differ from density functional theory calculations. They then conclude that 'prediction [by ab initio calculation] of the full elasticity tensor for Ti(3)SiC(2) has not been successful'. However the authors do not compare with previous experimental work in which Finkel et al measure the elastic moduli (Finkel et al 2000 J. Appl. Phys. 87 1701). The predictions of ab initio calculations (Yu et al 2005 J. Mater. Res. 20 1180) agree with the measurements of Finkel et al as well as with most other experimentally measured elastic moduli for MAX phases (Cover et al 2008 10 935; Cover et al 2009 J. Phys.: Condens. Matter 21 305403). The unrealistically high value of the C(44) constant obtained by Kisi et al, which would mean that Ti(3)SiC(2) is almost as resistant to shear as diamond, undermines confidence in their results.
  Journal of Physics Condensed Matter 07/2011; 23(26):268001; discussion 268002. · 2.55 Impact Factor
• Article: Hidden stressors in the clonogenic assay used in radiobiology experiments.

  M D E Potter, N Suchowerska, S Rizvi, D R McKenzie
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  ABSTRACT: While clonogenic assays are extensively used in radiobiology, there is no widely accepted procedure for choosing the composition of the cell culture media. Cell line suppliers recommend a specific culture medium for each cell line, however a researcher will frequently customize this aspect of the protocol by supplementing the recommended support medium with additives. For example, many researchers add antibiotics, in order to avoid contamination of cells and the consequent loss of data, with little discussion of the influence of the antibiotics on the clonogenic survival of the cells. It is assumed that the effect of any variables in the growth medium on cell survival is taken into consideration by comparing the survival fraction relative to that of controls grown under the same conditions. In the search for better cancer treatment, the effect of various stressors on clonogenic cell survival is under investigation. This study seeks to identify and test potential stressors commonly introduced into the cell culture medium, which may confound the response to radiation.
  Australasian physical & engineering sciences in medicine / supported by the Australasian College of Physical Scientists in Medicine and the Australasian Association of Physical Sciences in Medicine 06/2011; 34(3):345-50. · 0.56 Impact Factor
• Article: Dynamic modeling of lung tumor motion during respiration.

  E Kyriakou, D R McKenzie
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  ABSTRACT: A dynamic finite element model of the lung that incorporates a simplified geometry with realistic lung material properties has been developed. Observations of lung motion from respiratory-gated computed tomography were used to provide a database against which the predictions of the model are assessed. Data from six patients presenting with lung tumors were processed to give sagittal sections of the lung containing the tumor as a function of the breathing phase. Statistical shape modeling was used to outline the diaphragm, the tumor volume and the thoracic wall at each breathing phase. The motion of the tumor in the superior-inferior direction was plotted against the diaphragm displacement. The finite element model employed a simplified geometry in which the lung material fills a rectangular volume enabling two-dimensional coordinates to be used. The diaphragm is represented as a piston, driving the motion. Plots of lung displacement against diaphragm displacement form hysteresis loops that are a sensitive indicator of the characteristics of the motion. The key parameters of lung material that determine the motion are the density and elastic properties of lung material and the airway permeability. The model predictions of the hysteresis behavior agreed well with observation only when lung material is modeled as viscoelastic. The key material parameters are suggested for use as prognostic indicators of the progression of disease and of changes arising from the response of the lung to radiation treatment.
  Physics in Medicine and Biology 05/2011; 56(10):2999-3013. · 2.83 Impact Factor
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  Available from: Roberto Sangines

  Article: Optimizing efficiency of Ti ionized deposition in HIPIMS

  A E Ross, R Sanginés, B Treverrow, M M M Bilek, D R McKenzie
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  ABSTRACT: The evolution of plasma composition, electron temperature and deposition rates of high-power impulse magnetron sputtering (HIPIMS) of a titanium target have been studied by optical emission spectroscopy and deposition on a crystal oscillator, for pulse lengths of 50μs and 100μs over a range of voltages. The observations indicate a strong power dependence of the populations of Ti1+ ions with respect to the Ti neutrals. A clear saturation point for Ti1+ is seen, while the more highly ionized species increase, even at a constant electron temperature. The creation of the more highly ionized species follows the current, increasing throughout the pulse, and is attributed to ionization by collisions with energetic secondary electrons. Within the HIPIMS regime, deposition energy efficiency correlates positively with the emission from Ti1+ ions and negatively with emission from more highly ionized species.
  Plasma Sources Science and Technology 04/2011; 20:035021. · 2.52 Impact Factor
• Article: Energetic deposition of carbon in a cathodic vacuum arc with a biased mesh

  A. Moafi, D. W. M. Lau, A. Z. Sadek, J. G. Partridge, D. R. McKenzie, D. G. McCulloch
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  ABSTRACT: Carbon films were deposited in a filtered cathodic vacuum arc with a bias potential applied to a conducting mesh mounted in the plasma stream between the source and the substrate. We determined the stress and microstructural properties of the resulting carbon films and compared the results with those obtained using direct substrate bias with no mesh. Since the relationship between deposition energy and the stress, sp2 fraction and density of carbon are well known, measuring these film properties enabled us to investigate the effect of the mesh on the energy and composition of the depositing flux. When a mesh was used, the film stress showed a monotonic decrease for negative mesh bias voltages greater than 400V, even though the floating potential of the substrate did not vary. We explain this result by the neutralization of some ions when they are near to or passing through the negatively biased mesh. The microstructure of the films showed a change from amorphous to glassy carbonlike with increasing bias. Potential applications for this method include the deposition of carbon films with controlled stress on low conductivity substrates to form rectifying or ohmic contacts.
  Journal of Applied Physics 04/2011; 109(7):073309-073309-6. · 2.17 Impact Factor
• Article: Design of shallow acceptors in ZnO through early transition metals codoped with N acceptors

  X. M. Duan, C. Stampfl, M. M. M. Bilek, D. R. McKenzie, Su-Huai Wei
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  ABSTRACT: We perform first-principles density-functional theory calculations to study the atomic and electronic properties of early transition metals (Zr, Ti, Y, and Sc) codoped with N in wurtzite ZnO. By incorporating early transition metals Ti, Zr, Y, and Sc with N into ZnO simultaneously, we find that forming complexes (Zr-2N), (Ti-2N), (Y-N), and (Sc-N) induces fully occupied impurity bands with the N 2p character above the valence band maximum of host ZnO. With further doping of N in ZnO, the systems (Zr-2N):N, (Ti-2N):N, (Y-N):N, or (Sc-N):N have acceptor ionization energies lower than that of the isolated N acceptor in ZnO. Under different growth conditions (i.e., using an N2O or NO source for the nitrogen atoms), we calculate the formation energies of the defect complexes and compare the dopability of the selected codoped systems. Our results show that the valence band maximum characteristic of ZnO can be altered by compensated donor-acceptor pairs, thus improving the p-type dopability.
  Phys. Rev. B. 02/2011; 83(8).
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  Available from: M.M.M. Bilek

  Article: Fizeau interferometer system for fast high resolution studies of spectral line shapes.

  O Novák, I S Falconer, R Sanginés, M Lattemann, R N Tarrant, D R McKenzie, M M M Bilek
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  ABSTRACT: A monochromator∕Fizeau interferometer∕intensified CCD camera system is described that was developed for the measurement of the shape of spectral lines that are rapidly time varying. The most important operating parameter that determines the performance of the instrument is the size of the entrance aperture as this determines both the light throughput and the effective interferometer wavelength resolution. This paper discusses, both theoretically and experimentally, the effect of the finite source area on the instrumental resolution to assist in optimizing the choice of this parameter. A second effect that often produces a practical limit to the quality of the spectra is drift of the interferometer plates. Measurements of the shapes of spectral lines of ions and atoms ejected from the cathode spot of continuous and pulsed cathodic arcs are presented to demonstrate the utility of this instrument.
  The Review of scientific instruments 02/2011; 82(2):023105. · 1.52 Impact Factor